Various proposals have been made to change the timing of opening the intake valve in relation with the top dead center of the piston movement depending on the rotational speed of the engine so that the output property of the engine may be improved over a wide rotational speed range.
For instance, according to the proposal made in Japanese patent laid open (kokaki) publication No. 63-106310, a plurality of cams for producing different timings and total lifts are provided on a common camshaft for each cylinder, and are actuated by individual rocker arms rotatably supported on a common rocker shaft. The adjacent ones of these rocker arms can be selectively connected to each other so that the valve timing and the total lift of the corresponding valve may be changed in a stepwise fashion. According to the proposal made in Japanese patent laid open (kokai) publication No. 5-10161, the phase relationship between the rotational angles of the camshaft and the crankshaft can be varied in a continuous manner. Japanese patent (kokoku) publication No. 5-43847 discloses a combination of these two technologies.
However, when the phase relationship of the camshaft in relationship with the crankshaft is changed in the arrangement where a plurality of cams are provided for each cylinder to the end of varying the opening timing and the total lift of the intake valve, it is possible that the piston at the top dead center may interfere with the intake valve if the timing and the lift of the intake valve are not properly selected. Such an interference is highly destructive, and should be avoided by all means. As shown in FIG. 12, when the cam lift is relatively small, even if the cam phase is advanced or the opening timing of the intake valve is advanced, the intake valve would not interfere with the piston at the top dead center. However, as shown in FIG. 13, when the cam lift is relatively large, if the cam phase is advanced or the opening timing of the intake valve is advanced as indicated by the solid line, the intake valve may interfere with the piston at the top dead center.